Police are deploying "several hundred" more officers to secure this weekend's London Marathon in the wake of the bombings at the Boston race, Scotland Yard said on Friday.
Although there is no suggestion of a link between the US attack and the London race on Sunday, police said more search dogs would also be used to reassure the public.
"I have increased the number of officers for reassurance patrols by several hundred," Chief Superintendent Julia Pendry said.
"It's about making sure that people who come to London on Sunday feel safe when they are in the city.
"It is a good 40 percent increase on last year on numbers of officers on the ground. We've got more search dogs out, we have got more on high visibility patrols."
Boston police on Friday killed one of the marathon bombing suspects in a shootout and launched a massive manhunt for a second accused of being behind Monday's attack, which killed three people.
Pendry said Scotland Yard was in constant contact with US authorities over the attack.
"We're in touch with the Americans all the time," she said.
She added: "I would stress there is no change to the threat level to London and nothing at this stage to link the Boston bombings to the London marathon.
"Nevertheless we want to do all we can to help provide a secure environment in which the runners, spectators and volunteers can enjoy themselves."
The course snakes alongside the River Thames and passes iconic landmarks including Tower Bridge, the Houses of Parliament and Buckingham Palace.
Runners have been encouraged to wear black ribbons and organisers have announced that Â£2 ($3, 2.30 euros) for every finisher will be donated to a fund for victims of the Boston attacks.